Wiring boards have been heretofore produced by the use of copper-clad laminates in which a copper foil is laminated onto a surface of an insulating film such as a polyimide film using an adhesive.
Such a copper-clad laminate is produced by thermally bonding a copper foil under pressure to an insulating film having an adhesive layer formed on the surface. In the production of the copper-clad laminate, therefore, the copper foil must be handled alone. However, the tensile strength of the copper foil is lowered as the thickness thereof is decreased, and the lower limit of the thickness of the copper foil which can be handled alone is about 9 to 12 μm. If a copper foil having a thickness smaller than this thickness is used, handling of the copper film becomes very troublesome, for example, a copper foil with a support needs to be used. Further, if a wiring pattern is formed using a copper-clad laminate in which such a thin copper foil is bonded to a surface of an insulating film with an adhesive, warpage deformation of the resulting printed wiring board is brought about by heat shrinkage of the adhesive that is used for bonding the copper foil. In particular, with miniaturization and lightening of electronic equipments, thinning and lightening of printed wiring boards have been also promoted, and it is becoming impossible that the copper-clad laminates of three-layer structure consisting of an insulating film, an adhesive and a copper foil meet such printed wiring boards.
Then, instead of the copper-clad laminates of three-layer structure, laminates of two-layer structure in which a metal layer is directly laminated onto a surface of an insulating film have been employed. Such a laminate of a two-layer structure is produced by depositing a seed layer metal on a surface of an insulating film, such as a polyimide film, by means of electroless plating, deposition, sputtering or the like. On a surface of the metal thus deposited, copper or the like is deposited by plating, then a photoresist is applied, exposed to light and developed, and then etching is carried out, whereby a desired wiring pattern can be formed. In particular, the laminate of a two-layer structure is suitable for producing an extremely fine wiring pattern having a wiring pattern pitch width of less than 30 μm because the metal layer (e.g., copper layer) is thin.
By way of further background, Japanese Patent Laid-Open Publication No. 188495/2003 (JP '495) discloses a process for producing a printed wiring board comprising subjecting a metal coated polyimide film, which has a first metal layer formed on a polyimide film by a dry film-forming method and a second metal layer having conductivity that is formed by plating on the first metal layer, to etching to form a pattern, wherein after the etching, the etched surface is subjected to cleaning treatment with an oxidizing agent. In Example 5 of JP '495, an example comprising plasma depositing a nickel-chromium alloy in a thickness of 10 nm and then depositing copper in a thickness of 8 μm by plating is shown.
JP '495 further discloses that after the pattern is formed by etching, the first metal layer present on the surface of the polyimide resin film is treated with an oxidizing agent. In such a treatment using an oxidizing agent, however, a considerable amount of the first metal remains, and passivation thereof is not carried out completely, so that in severe environments short-circuit or the like sometimes occurs in a relatively short period of time. Further, the metal that forms the first metal layer in JP '495 is all treated with an oxidizing agent, but a part of the metal is hardly oxidized, and in this case, there is a possibility that favorable insulation is not formed between wiring patterns. In the treatment adopted in JP '495, moreover, a trace amount of a metal remaining is hardly passivated. Therefore, after a voltage is continuously applied for, for example, 1000 hours or more, the value of insulation resistance between wiring patterns tends to become lower than that before the application of a voltage though the value of insulation resistance is favorable immediately after the production.
Paragraphs [0004] and [0005] of Japanese Patent Laid-Open Publication No. 282651/2003 (JP '651) disclose that a metal layer 1 made of an alloy of copper and a metal other than copper is provided on a surface of a flexible insulating film 2 in order to ensure adhesion strength between the flexible insulating film and a wiring pattern, then on a surface of the metal layer 1 a copper foil is arranged to form a composite, and from the composite a flexible wiring board is produced. JP '651 further describes that at the peripheral lower part of the lead of the wiring pattern formed by the use of such a composite, the metal layer 1 remains as an unremoved part as shown in FIG. 5 of JP '651, and further discloses that because of the unremoved part, abnormal deposition 6 of the plating metal takes place. Moreover, JP '651 further discloses that at the place of the abnormal deposition 6 of the plating metal, a crystal of tin grows and becomes a “whiskers”, and because of the whisker, a short-circuit takes place in the wiring pattern. That is to say, if the metal layer 1 provided to ensure adhesion strength of the wiring pattern is left as it is and if a tin plating layer is formed on the surface of the metal layer 1, a whisker is generated from the thus formed tin plating layer. In JP '651, therefore, the metal layer 1 is completely removed as described in paragraph [0023] thereof.
However, it is extremely difficult to completely remove the metal layer 1 from the outer periphery of the wiring pattern. In the process described in JP '651, the metal layer 1 remains as it is at the lower part of the outer periphery of the wiring pattern though the amount is small, and generation of whiskers from the tin plating layer attributable to the residual metal layer 1 cannot be completely prevented.